This invention relates to a cutting tool for interrupted turning use.
As a material for cutting tools, there is commonly used cemented carbide composed of a ceramics phase (such as carbide or carbo-nitride) and a metal phase (such as cobalt and nickel) when a steel member is cut by a cemented carbide tool, the metal phase adheres to the steel member, so that the cemented carbide tool is liable to wear.
For this reason, conventionally, crystalline or amorphous ceramics which can not easily adhere to steel is coated on a cemented carbide tool by chemical vapor deposition (CVD) or physical vapor deposition (PVD). In the formation of such ceramic coating, care is taken to prevent the generation of internal defects, such as voids and cracks, as much as possible. The tool coated by means of a CVD process is superior in wear resistance but is inferior in fracture resistance to the tool coated by means of a PVD process. The reason for this is as follows.
The processing temperature in the CVD process is so high that the diffusion occurs between the coating layer and the substrate to provide sufficient adhesion. Therefore, the wear resistance of the cemented carbide tool coated by CVD process is particularly excellent. However, cracks produced in the deposited crystalline or amorphous ceramics, propagate into the substrate, and therefore there is encountered a disadvantage that the cutting tool coated by a CVD process is liable to occur fracture. According to a literature, "Cemented Carbides and Sintered Hard Materials" Hisashi Suzuki, by Maruzen Publish. Co. (Tokyo), 1986, p. 218. Deposition of ceramics lowers the rupture strength by 50%. For the purpose of improving the rupture strength of the coating layer, extensive study has been made with respect to coating conditions related to the thickness of the coating layer, the particle size of its crystals and its crystalline structure, as well as a heat treatment after coating. However, satisfactory results have not yet been achieved. The fracture resistance decreases with increase of the thickness of the coating layer, and for this reason, the thickness of the coating layer of the existing cutting tool is in the range of between several micron (.mu.m) and about 10 .mu.m. On the other hand, the wear resistance can be enhanced in proportion to the thickness of the ceramic coating, and therefore if any method of improving the fracture resistance of the coating layer is found, it will become possible to develop a tool which has a further greater coating thickness and hence is more excellent in wear resistance.